A man smokes near the Forbidden City on a heavy haze day in central Beijing January 13, 2013. Air quality in Beijing was the "worst on record" on Saturday and Sunday, according to environmentalists, as the city's pollution monitoring centre warned residents to stay indoors with pollution 30-45 times above recommended safety levels. REUTERS/Jason Lee (CHINA - Tags: ENVIRONMENT SOCIETY)

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REUTERS

Soot from diesel engines and coal smoke was a main culprit in the recent Beijing smog crisis. Now a new report says soot is also a much bigger contributor to global warming than had been thought. Host Marco Werman gets the latest on soot from The World's environment editor Peter Thomson.